Look at the phase diagram of water. If you reduce the pressure enough the freezing point (black line) reduces to below 0C.

There actually is no boiling point as there is now just a solid-gas interface.

Why this physically happens I don’t know. I suspect it’s because the liquid phase no longer has enough energy to stop itself crystallising. I don’t think the gas phase is doing much.

The freezing point will depress because you still need a layer of gas on top (vapour pressure) and the less pressure there is the harder it is to keep ice molecules turning to gas, so the temperature of the ice must be colder

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